1. Field of the Invention
The present invention generally relates to an exposure apparatus, and more specifically, it relates to an exposure apparatus that performs alignment of an original and a substrate with high precision.
2. Description of the Related Art
Along with the recent remarkable development of device (semiconductor devices, liquid crystal devices, and so forth) manufacturing techniques, the progress of micropatterning techniques is also prominent. Particularly in the lithography process, exposure apparatuses having submicron resolutions are mainstream. For higher resolutions, a larger numerical aperture (NA) of a projection optical system and a shorter exposure light wavelength are being realized.
There are exposure apparatuses in which the space between a wafer and a projection optical system is filled with liquid, and the wafer is exposed through the liquid in order to improve resolution and focal depth.
With the improvement of resolution of projection optical systems, high precision is required for the relative alignment between a wafer and a reticle (mask) in an exposure apparatus. That is to say, exposure apparatuses are required to have a function as a high-precision position detecting apparatus.
High throughput is also an important function of exposure apparatuses. Twin stage type exposure apparatuses, which have two stages, achieve this function.
Twin stage type exposure apparatuses have at least two spaces, a measurement space where the position of a wafer is detected, and an exposure space where the exposure is performed based on the measurement result. The two stages are alternated between the measurement space and the exposure space.
In the measurement space is provided an alignment detection system, which optically detects an alignment mark on the wafer. The positional information of the alignment mark is obtained from the alignment detection system, and the exposure position of the wafer in the exposure space is determined. When a stage moves from the measurement space to the exposure space, the position of the stage needs to be controlled. Therefore, on each stage is disposed a fiducial mark.
In the measurement space, the fiducial mark is detected by the alignment detection system, and the relative position of the alignment mark on the wafer to the fiducial mark is measured. Thereafter, the stage moves to the exposure space. The relative positional relationship between the reticle and the fiducial mark is detected in the exposure space. The relative positional relationship between the measurement space and the exposure space is thereby ensured. Therefore, in twin stage type exposure apparatuses, it is necessary to detect the fiducial mark on the stage in the two spaces, the measurement space and the exposure space.
After completion of exposure of the wafer, the stage is moved to the measurement space, and the position detection of the next wafer and the position detection of the fiducial mark are performed. As described above, in the case where a plurality of wafers are exposed, the detection of the fiducial mark is repeatedly performed in the measurement space, then in the exposure space, then in the measurement space.
A method for detecting the position of a fiducial mark in the exposure space is proposed in US2005/0146693 and Japanese Patent Laid-Open No. 2005-175400. In the method, a fiducial mark is used that has portions transmitting exposure light (light-transmitting portions) and portions opaque to exposure light (light-shielding portions), and the position is detected from the amount of light passing through the light-transmitting portions. On the reticle is provided a mark similar to the fiducial mark, and the mark is illuminated with exposure light. An image of the mark on the reticle is formed on the fiducial mark on the wafer stage by a projection optical system. The position of the fiducial mark in the optical axis direction of the projection optical system and the directions perpendicular to the optical axis is changed relative to the image of the mark on the reticle. Thereby, the amount of the exposure light passing through the light-transmitting portions of the fiducial mark changes. From the change profile, the relative positional relationship between the reticle and the wafer stage can be measured.
Such relative alignment between the reticle and the wafer stage can be used not only in twin stage type exposure apparatuses but also in conventional single stage type exposure apparatuses. In that case, it is used for measuring the relative positional relationship (so-called base line) between an off-axis alignment detection system detecting an alignment mark on a wafer and a projection optical system.
From the viewpoint of improving the throughput, the amount of time for measuring the relative positional relationship between the reticle and the wafer stage or measuring the base line needs to be minimized.
Particularly in twin stage type exposure apparatuses, since the measurement of the fiducial mark needs to be performed for every wafer, the amount of time for measuring the relative positional relationship between the reticle and the wafer stage significantly influences the throughput.
The rotation component of the reticle and the wafer and the magnification component of the reticle can also be measured using the fiducial mark. When these components are measured, however, the throughput is also reduced.
FIGS. 13A to 13C schematically show marks (calibration marks) on a reticle and fiducial marks of a wafer stage according to a conventional art. On the reticle 2 are provided, as shown in FIG. 13A, calibration marks 601 and 603 for measuring the position in the X direction and calibration marks 602 and 604 for measuring the position in the Y direction.
FIG. 13C shows fiducial marks provided on the wafer stage and viewed from the Z direction (the optical axis direction of the projection optical system). On the wafer stage are provided a fiducial mark 605 corresponding to the calibration marks 601 and 603, and a fiducial mark 606 corresponding to the calibration marks 602 and 604.
FIG. 13B schematically shows the fiducial marks viewed from a direction perpendicular to the optical axis. In FIG. 13B, light-transmitting portions (openings) 32a and 32b, which are formed through a light-shielding portion 31, correspond to the fiducial marks 605 and 606, respectively. Light beams passing through the light-transmitting portions 32a and 32b are incident on the photoelectric conversion elements 30a and 30b. The photoelectric conversion elements 30a and 30b measure the amounts of the light beams. Even if light beams fall on the light-transmitting portions 32a and 32b simultaneously, the photoelectric conversion elements 30a and 30b can separately detect the light beams.
As described above, according to the conventional art, two pairs of calibration marks 601 and 602, and 603 and 604 corresponding to the fiducial marks 605 and 606 are provided on the reticle 2. A pair of fiducial marks 605 and 606 is moved to measure the position of each of the two pairs of calibration marks on the reticle 2. Therefore, the pairs of calibration marks 601 and 602, and 603 and 604 on the reticle 2 are configured so that the two kinds of marks are arranged in the same order corresponding to the order of arrangement of the fiducial marks 605 and 606.
The rotation component of the reticle 2 is measured, for example, from the difference value between the positions in the Y direction of the Y direction marks 602 and 604 provided in two places on the reticle 2. When the distance between the two Y marks is L, and the difference value is ΔY, the rotation component θ=ΔY/L. Therefore, L is maximized to improve the measurement precision.
In the conventional art, the same kinds of marks 602 and 604 are provided on the left and right of the reticle, and the wafer stage is driven to measure the rotation component with the corresponding fiducial mark 606. Therefore, the measurement takes time. In addition, the fiducial marks 605 and 606 need to be disposed so that the distance therebetween is greater than or equal to the size of the photoelectric conversion elements 30a and 30b. Therefore, the distance L between the calibration marks 602 and 604 provided on the reticle 2 is small. As described above, when the distance L is small, the measurement precision of the rotation component is prevented from being improved.